Star clusters form from the same molecular cloud. An
open cluster of stars is one that can contain about
a dozen to a few hundred members. In addition, they
are spaced loosely and are generally younger, hot
stars.

The Pleiades open cluster is a group of newly
formed B-type stars. This 115 million year-old open cluster is easily visible with the naked eye. The brightest stars have a surface temperature of 30,000 Kelvin, however there are other stars that are a part of the cluster that have a variety of temperatures much cooler than these.

Star clusters give
astronomers a unique opportunity. All we know about
stellar evolution come from star clusters (both open
and closed). Stars in a cluster form at different
times because of a variety of masses, but all
contain nearly the same
spectroscopic data. By
careful observation, a
Color-Magnitude Diagram can
be created to plot the progression of evolution. A
Color-Magnitude Diagram (or CMD) is really a
specialized H-R diagram:

(Image credit: Brooks/Cole Thomson Learning)

The
organization of stars on the diagram give us the age
of the cluster. By evaluating when stars enter the
later stages of evolution - called the Main-Sequence
Turn Off Point - we can determine the age of the
cluster.

Sometimes the mutual gravity of a
cluster cannot hold the cluster together. This
results in an stellar association. The most common
member of a stellar association are O and B type
stars. As a result, the correct term is OB
Association.